The method of the invention relates to the treatment of domestic wastewaters and more particularly to increasing the nitrogen content of dried sludge derived from such wastewater treatment.
In the treatment of domestic wastewaters by conventional means, including primary sedimentation, biological oxidation, chemical precipitation, biological nitrification and denitrification, sludge streams containing relatively high concentrations of solids are produced. These sludges will commonly contain about 65 percent of the biochemical oxygen demand (BOD) and some 20 percent of the nitrogen which was in the untreated wastewater. Because of the high pollutional potential of such sludges, they must be extensively treated before safe, economical disposal of the solids is possible. Several methods have been suggested for the stabilization of such sludges. The process most commonly used, however, consists of first removing as much water from the sludge as possible by gravity or flotation thickening. The sludge is then biologically digested under anaerobic conditions. This digestion results in the conversion of much of the putrescible organic material to carbon dioxide and methane, the latter of which may be used as fuel. During this process, much of the nitrogen contained in the sludge is converted to water soluble ammonia.
After digestion, water in the form of digester supernatant is removed by allowing the sludge to stand quiescent for a period of time in the digester and decanting off the liquid. The concentrated sludge is then dewatered still further by vacuum filtration or other means. Both the digester supernatant and liquors from the dewatering process are commonly returned to the primary clarifiers as these streams contain significant quantities of organic material and, therefore, require treatment in the wastewater treatment processes.
The sludge solids which remain after filtration may be disposed of by incineration, land filling, composting or drying for sale as a soil conditioner. While the utilization of sludge as a soil conditioner or fertilizer has obvious advantages, many plants attempting to market dried or composted sludge have experienced difficulty. This relates to the conversion of organic nitrogen to water soluble ammonium salts within the anaerobic digester, which results in much of the nitrogen originally contained in the sludge being removed with the water in the digester supernatant and dewatering liquors. This ammonia nitrogen is returned with these streams to the head of the wastewater treatment plant to be removed as valueless nitrogen gas if denitrification is practiced, or to be discharged in the plant effluent.
In plants practicing nitrogen removal, return of these streams may substantially increase the cost of treatment, since they contain as much as 20 percent of the total nitrogen received by the plant. Further, the digester supernatant and sometimes the vacuum filtrates may be very odorous and further, may increase the organic (BOD) loading on the facilities.
I have discovered a method of treatment for digester supernatant and liquors from digested sludge dewatering by which the nitrogen contained in these streams is recovered as ammonia or a salt thereof, the streams are effectively deodorized and their BOD content substantially reduced. The nitrogen so recovered may be mixed with sludge dried in conventional drying equipment for the production of a high-grade organic fertilizer. Thus, benefits are gained both from the reduction of nitrogen, odor, and BOD of the digester supernatant and in the disposal of sludge solids by converting such materials to a saleable product.
While there are several known methods for recovering ammonia from ammoniacal liquors obtained during the treatment of fecal matter, such as U.S. Pat. Nos. 325,771 and 918,744, all of such methods involve one or more difficulties which greatly increase the cost of the removal process to the point where it is not economically competitive with my method. In the method disclosed in U.S. Pat. No. 325,771 (Schneider) the ammoniacal liquor is mixed with lime or other suitable alkaline substances and is heated by steam, acting either directly or through the walls of pipes, to distill the ammonia freed by the alkaline agent. This process is undertaken at either atmospheric pressure or an elevated pressure with the result that the ammonia gas, which is soluble in the water, is not efficiently freed in gaseous form. A similar problem is true in the U.S. Pat. No. 918,744 reference. The use of elevated pressure in such distilling operations has heretofore been considered to be an important feature. As, for example, see U.S. Pat. No. 1,838,587 which specifically teaches that a reduced pressure during such distilling operations results in a substantial lowering of the rate of the chemical reaction with a loss of efficiency.
In the practice of wastewater treatment, lime or other alkali is commonly used to increase the alkalinity of wastewater or to precipitate phosphorus. One installation has even been constructed wherein lime is added to the entire waste system, both to precipitate phosphorus and to free ammonia. In this system, the wastewater stream is passed through a tall tower after the lime addition. Air is then passed countercurrent to the waste to strip the liberated ammonia according to a well known process. This practice requires that the entire wastewater flow stream be treated rather than the relatively small streams from the digester and dewatering devices. The process is operated at the ambient temperature and pressure with a consequent loss of efficiency in carrying out the reaction. A further difficulty is that the process utilizes air stripping with a consequent release of the ammonia gas to the atmosphere rather than its recovery. It also creates a potential air pollution problem.
It has been the practice in refining of oil to produce gasoline and other products to strip "sour water," that is, condensate water which has been in contact with crude oil or partially refined petroleum, by steam stripping the sour water to remove hydrogen sulfide and ammonia. While sulfur is often recovered from the gas so liberated the ammonia which is also removed along with the sulfur is typically destroyed by burning.
Coke plants also commonly strip sour water with steam; however, the intent of this treatment is to remove both ammonia and hydrogen sulfide from the sour water and the released gas is burned rather than recovering the ammonia. Such conventional stripping as practiced in either oil refining or coal coke plants is not applicable to domestic wastewaters because of the high levels of dissolved carbonates in these streams. A high carbonate content results in scaling of the surfaces.
Still another prior art device has been described for the removal of ammonia from digester supernatant and other such streams. In this device, the liquors are mixed with lime and passed directly to a stripping column. Air is introduced through a blower at the bottom of the column to serve as the stripping medium. The ammonia-air mixture is then discharged to the atmosphere at the top of the vessel. Such treatment does not provide for the recovery of ammonia. The device further operates at ambient temperatures and pressures, thus reducing its efficiency. Finally, the device described has not been successfully applied commercially to the treatment of wastewaters because of severe scaling in the column.